Thermal process for the conversion of uranium hexafluoride

ABSTRACT

A single-step process for producing solid uranium oxide and gaseous HF from UF 6  which comprises bringing together two gaseous reactant streams, one of said streams comprising UF 6  optionally admixed with oxygen as O 2 , and the second reactant stream comprising a mixture of hydrogen as H 2  or as a hydrogen-containing compound and oxygen as an oxygen-containing compound, said gaseous reactant streams being brought together at a temperature and composition such that the UF 6  is converted rapidly by flame reaction into readily separable solid uranium oxide and a gaseous HF product.

RELATED APPLICATION

This is a continuation-in-part of U.S. application Ser. No. 08/635,190,filed Apr. 19, 1996, the contents of which are incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates to a thermal process for the conversion ofuranium hexafluoride (UF₆) to give solid uranium oxide (essentially UO₂)and hydrogen fluoride (HF). The process represents an efficient way ofconverting UF₆ to UO₂ with recovery of the attendant fluorine as HF by asingle-step procedure involving two gaseous feed streams.

BACKGROUND TO THE INVENTION

The separation of uranium isotopes for defense needs and the preparationof commercial nuclear fuels have mainly been by processes that produceenriched and depleted uranium (i.e., enriched or depleted in theuranium-235 isotope) as UF₆. Much of the enriched UF₆ is converted toUO₂ by processes selected to give the ceramic sinterability needed forpreparation of nuclear fuel pellets. The much larger amounts of depletedUF₆ from the enrichment process are mostly stored as solid UF₆ in steelcylinders.

As of July, 1995, the U.S. Department of Energy had over 600,000 MT ofdepleted UF₆ (containing over 400,000 MT of uranium) stored inapproximately 66,000 cylinders. Long term strategies for management ofthis depleted uranium require conversion of the UF₆ to uranium oxides.However, while procedures for converting UF₆ to uranium oxides areknown, the currently available procedures are not particularly efficientor economical for converting depleted UF₆ to solid uranium oxides,notably UO₂, suitable for disposal, storage, or further use. Morespecifically, the UF₆ conversions for nuclear fuels were developed toprepare UO₂ with well controlled ceramic properties and are not optimumfor the much larger-scale conversions of depleted uranium. Furthermore,because of the need to control their ceramic properties and thethermodynamic limitations, the known commercial conversion processes arecomplex with multiple process stages and include the formation ofintermediate solids such as UO₂ F₂ or UF₄. Additionally, the fluorineby-products of these conversion processes are usually radioactive wasteswith high disposal costs.

Uranium oxides are thermally and chemically stable, non-volatile andessentially insoluble in rain and ground water, and are the preferredcompositions of uranium for long term storage or disposal. The moststable oxide in the environment is U₃ O₈, but other oxides ranging fromUO₂ to U₄ O₉ and UO₃ and combinations of oxides are also acceptableproducts. Uranium and fluorine are very reactive elements chemicallyand, the conversion of UF₆ to uranium oxides also produces a fluorinecompound as a product. While some fluorine compounds have littlecommercial value and represent waste products with consequent disposalcosts, hydrogen fluoride is a valuable commercial chemical with manyuses to provide a market for the fluorine in the depleted UF₆.

The conversion of UF₆ into uranium oxides and HF requires reaction withoxygen as O₂ or compounds containing oxygen and with hydrogen as H₂ orcompounds containing hydrogen. One process used to prepare nuclear fuelUO₂ from enriched UF₆ uses gross excesses of water vapor and H₂ in twosteps with the two principal reactions being:

    UF.sub.6 (g)+2H.sub.2 O(g)≧UO.sub.2 F.sub.2 (s)+4HP(g)(1)

    and

    UO.sub.2 F.sub.2 (s)+H.sub.2 (g)≧UO.sub.2 (s)+2HF(g)(2)

where (g) and (s) represent, respectively, gas and solid. The preferredsequence is to perform reaction (1) at about 250° C. followed byreaction (2) at about 650° C.

The complete conversion of the uranium solids to UO₂ as represented byreaction (2) requires good contact of the solids and gas over arelatively long period of time, on the order of an hour or more. Otherknown conversion processes are precipitation of ammonium diuranate orammonium uranyl carbonate from reaction of UF₆ with aqueous solutions.All of these processes require multiple steps and give dilute aqueousfluoride solutions of little or no value as the fluoride product.Nevertheless, these conversion reactions are chosen because they givethe resulting uranium oxide the ceramic properties needed forfabrication of nuclear fuels.

Numerous U.S. patents have been issued directed towards processes forthe conversion of UF₆ to uranium oxides. See, for example, U.S. Pat. No.4,830,841 and the U.S. patents listed therein which describe proceduresfor converting UF₆ to uranium dioxide in furnaces, rotary kilns,fluidized beds or the like. As representative of such art, it is notedthat U.S. Pat. No. 4,830,841 itself is concerned with a process forpreparing UO₂ from UF₆ by reacting UF₆ with steam to produce submicronuranyl fluoride powder, fluidizing a bed of uranium oxide material witha mixture of steam, hydrogen and inert gas at about 580° C. to about700° C., and introducing the submicron uranyl fluoride powder into thefluidized bed of uranium oxide material so that the uranyl fluoridepowder is agglomerated, densified, fluidized, defluorinated and reducedto a fluoride-containing uranium oxide material which is removed fromthe fluidized bed and then contacted with hydrogen and steam at elevatedtemperature to obtain UO₂ essentially free of fluoride.

Another U.S. Pat. No. 3,260,575 describes the preparation of ceramic UO₂fuel material by a single-step process comprising the reaction of UF₆with a stoichiometric excess, generally at least 1.5 times thestoichiometric amount, and preferably much larger (e.g., 30-40 times thestoichiometric amount) of a gaseous mixture of hydrogen andoxygen-bearing gas at a temperature above 1100° C. and a pressure notexceeding 20 torr., i.e. 20 mm Hg absolute. The patent specifies that atemperature of at least 1100° C. is required to avoid the formation ofUF₄ and that a pressure less than 20 torr is critical, fluorideintermediates being produced along with UO₂ at higher pressures.

Another process directed towards the recovery of anhydrous hydrogenfluoride from UF₆ gas is disclosed in U.S. Pat. No. 5,346,684(equivalent to EP 529768 A1). That process involves reacting UF₆ in aprimary reactor with steam to produce a uranyl fluoride intermediate anda gaseous mixture of hydrogen fluoride and water. The uranyl fluoride isthen fed to a secondary reactor and reacted with water to produce a U₃O₈ product for disposal and a gaseous mixture of water, HF and oxygen.The gaseous mixtures from the two reactors are combined and thendistilled to obtain an anhydrous HF product. An azeotrope of water andHF is vaporized and returned to the primary reactor.

Another prior process for converting UF₆ to uranium oxides involvesfeeding the UF₆ into a molten metal bath where the UF₆ is broken downinto recoverable components including uranium oxide and HF.

Notwithstanding the extensive prior efforts referred to above, thereremains a substantial need for improved procedures for converting UF₆,particularly depleted UF₆, into solid UO₂ in a form suitable forstorage, disposal or use. The primary object of the invention is toprovide such a process. Other objects, including, for example, theprovision of HF in concentrated aqueous solution or in other highlyuseful form, such as anhydrous HF, will also be evident from thedescription of the invention which follows.

SUMMARY OF THE INVENTION

The invention provides a single-step process for efficiently convertingUF₆ into solid UO₂ and gaseous or condensed phase HF. The processinvolves bringing together two gaseous reactant streams, one of thestreams comprising UF₆ optionally admixed with oxygen as O₂, and thesecond reactant stream comprising a mixture of hydrogen as H₂ or as ahydrogen-containing compound and oxygen as an oxygen-containingcompound, the gaseous reactant streams being brought together at atemperature, pressure and composition such that the UF₆ is convertedrapidly by flame reaction into readily separable solid uranium oxide(essentially UO₂) and a gaseous HF product. While the composition of thetwo gaseous reactant streams can be varied, as discussed hereafter, careshould be taken to avoid using mixtures which might create an explosivepotential. For example, the second reactant stream preferably does notinclude the combination of H₂ and O₂ because of the possibility of anexplosion.

The present process is primarily intended for use with depleted UF₆.However, the process can also be used with natural assay or enrichedUF₆. In either case, a solid oxide consisting essentially of UO₂ and agaseous HF product are obtained. These are readily separated from eachother with the gaseous HF product being condensed to provide a highlyconcentrated aqueous HF solution or anhydrous HF. This fluoride productcan be used directly by the chemical industry for the manufacture offluorine (F₂) or replacement refrigerants (e.g.,non-chlorofluorocarbons) or by the uranium industry for the manufactureof UF₄ and UF₆. The solid UO₂, which is readily collectable, is easilyrecovered for storage or use.

The process is not sensitive to the pressure of the reactants and theoperating pressure of the system can be varied to accommodate sizing andthroughput requirements. However, it is particularly important to theusefulness of the process that the gaseous reactant streams are broughttogether and reacted at a pressure which is essentially atmospheric orabove, although in some circumstances, it may be desirable to operate ata pressure slightly below atmospheric.

An important distinguishing feature of the present process over priorprocedures is that it does not involve the use of fluidized beds, moltenmetal or the like to obtain the desired products. In essence, thepresent process simply involves bringing the two gaseous reactantstreams together so that a flame reaction occurs and collects theresultant solid oxide product and gaseous HF product.

DESCRIPTION OF PREFERRED EMBODIMENTS

Preferably the feed stream comprising UF₆ contains at least part of theoxygen needed for the reaction, the balance, if any, of the oxygen beingincluded in the second feed stream. The oxygen in the UF₆ feed streamshould only be in the form of O₂ to avoid the potential for prematurereactions. It is also preferred that the UF₆ feed stream does notcontain H₂.

The hydrogen in the second reactant stream can be in the form of H₂ oras a hydrogen-containing compound such as H₂ O, NH₃ and/or CH₄.Preferably all of the hydrogen needed for the reaction is in the secondfeed stream. Desirably, the second feed stream comprises a mixture of H₂and H₂ O or a mixture of H₂ and CO₂.

While the composition of each gaseous reactant stream can be varied,care must be taken in preparing each stream to avoid the possibility ofcreating an explosion potential or premature reaction. For example,undesirable mixtures include H₂ and O₂, UF₆ and H₂ O, UF₆ and H₂, NH₃and O₂, CH₄ and O₂. Typical useful mixtures include, without limitationthereto, UF₆ and O₂ ; (H₂ and/or NH₃) and H₂ O, (NH₃ and/or H₂) and CO₂; and CH₄ and H₂ O.

One or both feed streams may also include an inert gas such as argon ornitrogen.

The reaction can be carried out over a relatively wide temperature rangesuch as between 900° C. to 1500° C. or higher, e.g. up to 2000° C., witha temperature of around 1100° C. being generally preferred. The reactiontemperature or, stated otherwise, the temperature of the reactionproduct, can be controlled by varying the amount of oxygen in the UF₆feed stream and the flow rate of the gases used for the reaction.

The reaction can be carried out over a relatively broad pressure rangesuch as between atmospheric and 250 psia. It may be useful in some casesto operate at slightly below atmospheric, e.g. at about 10 to 13 psia,and possibly as low as 5 psia, but atmospheric or essentiallyatmospheric pressure is preferred. If a temperature in excess of 2000°C. is used, the pressure is preferably decreased below atmospheric tominimize the formation of undesired by-products (e.g. UF₄).

The ratio of hydrogen and oxygen to UF₆ can also be varied to controlthe amount of water vapor in the gaseous HF product. However, generallyspeaking, the amounts of oxygen and hydrogen in the feed gases, i.e.,the total amount of oxygen and hydrogen used in the process, should beless than about 1.5 times the stoichiometric amounts needed for reactionwith the UF₆ to form UO₂ and HF. Preferably the hydrogen and oxygen areused near (i.e., around 10-30% in excess of) the stoichiometric amounts.

A particularly unique feature of the invention is the finding that thedesired reaction to provide UO₂ and HF can be accomplished in a veryshort period of time. Typical reaction times are on the order offractions of a second although times in excess of this can be used.

Advantageously the process is carried out using a closed crucible orreactor made of suitable refractory material, e.g. graphite or alumina.High melting point nickel and nickel alloys that form stable metalfluorides can also be used. The gaseous reactant streams are fedseparately to the reactor. Within the specified temperature range of theinvention, the reactant streams react essentially spontaneously oncontact within the reactor UO₂ provide solid UO₂ and gaseous HF. The UO₂can be collected from the bottom of the reactor or filtered from thereactor off-gas while the HF may be withdrawn from multiple points, e.g.reactor top or side, and, if desired, condensed.

Advantageously, the reactant streams are fed into the reactor to obtainintimate mixing of the reactants such as through a concentric tubearrangement, with the UF₆ gas being fed through, for example, the innertube and the other gaseous reactant stream being fed through an outertube. The desired reaction occurs at the exit or discharge ends of theconcentric tubes where the reactant streams come together.Alternatively, the gaseous reactant streams can be fed to separateinlets of a nozzle arrangement rather than using concentric tubes, thereactant streams being brought together at or just before the nozzledischarge. In either case, one or both of the reactant streams can bepreheated, for example, to 700°-1000° C. The reaction does not requirethe application of additional heat as the overall reaction is anexothermic one. With appropriate selection of the composition of thereactant gas streams, an equilibrium reaction temperature of around900°-2000° C. is readily attained. Preheating of the reactant gasstreams is generally desirable, particularly if oxygen is not includedin the UF₆ reactant gas.

A suitable way of practicing the invention comprises the use of a burnerdesigned to receive UF₆ in one feed stream which may also contain O₂ andanother separate feed stream comprising H₂ O or CO₂ which also containsthe principal source of hydrogen as H₂, NH₃ or CH₄. The O₂ and H₂ Oand/or other oxygen-containing compounds together should preferablysupply a slight excess, e.g. 10 to 20 percent over the stoichiometricamount, of oxygen over that needed to form UO₂. The H₂ O and/or otherhydrogen-containing compounds together should also supply a slightexcess of hydrogen over that needed to form HF. The excess of oxygenforms CO if carbon is present and H₂ O if excess hydrogen is present.While a stoichiometric excess of hydrogen and/or oxygen is generallypreferred, it is recognized that sub-stoichiometric amounts of hydrogenand/or oxygen can also be used to provide a somewhat different uraniumoxide product distribution.

As noted earlier, the oxygen feed rate (when O₂ is admixed with UF₆) isused to control the temperature of the products of reaction. Withoutoxygen feed, the gases should preferably be preheated to control thetemperature of the products of reaction in the desired range specifiedfor the present process, i.e. 900°-2000° C. or higher.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is further described by reference to the accompanyingdrawings wherein:

FIG. 1 is a flow diagram showing one arrangement for practicing thepresent process;

FIG. 2 graphically shows equilibrium compositions versus producttemperature using a reaction arrangement as in FIG. 1;

FIG. 3 diagrammatically shows in more detail one way of supplying thereactant gas streams to the reaction zone;

FIG. 4 shows an alternative flow diagram for the practice of theinvention; and

FIG. 5 diagrammatically illustrates another arrangement for bringing thereactant streams together for reaction.

With more specific reference to FIG. 1, two gaseous reactant streams (1)and (2) are fed continuously to a lance or burner (3) comprising a pairof concentric tubes (4) and (5) positioned within the reaction vessel(6).

A pre-heated gaseous mixture of UF₆ and O₂, optionally including aninert gas such as N₂ or argon, is fed through the inner tube (4) while apre-heated gaseous mixture of H₂, NH₃ and/or CH₄ and H₂ O or CO₂,optionally with inert gas such as N₂ or argon, is fed through the outertube (5). Optionally, gaseous feeds are heated in the lance or burner(3) by an external heating source on vessel (6). The two gaseous streamsmix and essentially simultaneously react at the discharge ends of thetubes to form UO₂ and HF without any significant accumulation ofintermediate uranium compounds as solids. The reaction productsdischarge into the lower portion of vessel (6) which is preferablyprovided with means to facilitate separate discharge of gases andsolids. The uranium oxides are recovered at (7) as dry, granular solidssuitable for known methods of storage, disposal or use. The gaseousproducts collected at (8) are filtered (not shown) to remove entrainedsolids and cooled in condenser (9) to allow separation of condensed HFand residual H₂ O (10). By appropriate selection of the composition ofthe reactant streams, the amount of water formed in the reaction can becontrolled to provide the desired concentration of HF solution resultingfrom the condensation at (10). The resulting condensed product has manycommercial uses as collected in the form of highly concentrated aqueousHF or essentially anhydrous HF. The non-condensable gases (11) may bepartly recycled to feed (1) or (2) or they may be totally dischargedfrom the system for use or disposal.

FIG. 2 is discussed hereafter in connection with Example 2. The figure,as earlier noted, shows that the optimum results, with respect to theproduction of UO₂ and HF at atmospheric pressure, are realized at anequilibrium temperature in the range of 900° C. to 2000° C.,particularly around 1100° C.

FIG. 3 shows a feed apparatus for introducing a pre-heated feed gascontaining UF₆ and a second pre-heated feed gas containing compounds ofhydrogen and oxygen. The UF₆ feed (12) flows through an inner tube (14)selected to be corrosion resistant for UF₆ and O₂ and for all the othergaseous feeds. The other gaseous feed mixture (13) flows through theannulus between the inner tube and a second, larger tube (15) selectedto be corrosion resistant for HF and for the gaseous feed mixtures. Al₂O₃, nickel, and nickel alloy tubes can be used when they are passivatedby a film of solid fluorides. Alternatively, other fluoride compatiblematerials, such as CaF₂ and lanthanum hexaboride, may also be used.

A third, large, graphite tube (not shown) can be placed around the tube(15) so as to provide for an inert gas flow to prevent excessivetemperatures and, more importantly, avoid corrosion of the outer surfaceof tube (15). The UF₆ feed (12) includes a controlled supply of O₂ toregulate the reaction temperature and, optionally, N₂, argon or otherinert gas. The feed (13), as shown, comprises H₂, NH₃ or CH₄ and H₂ O orCO₂ as the hydrogen and oxygen sources. Optionally, N₂, argon or otherinert gas may also be included in feed (13). As in the case of thesystem shown in FIG. 1, the pre-heated gases issuing at the exits oftubes (14) and (15) flame react essentially simultaneously as they comein contact (see "Reaction Zone") to form the desired solid UO₂ andgaseous HF while leaving H₂ O and H₂ as excess reactants and inerts (Ar,N₂) in the embodiment as shown.

FIG. 4 shows an arrangement which is useful to avoid excessivetemperatures or larger heat fluxes. In this embodiment, the pre-heatedoxidizing feed gas (16) containing UF₆, O₂ and N₂ or argon and thepre-heated reducing feed gas (17) comprising a mixture of H₂, NH₃ or CH₄; H₂ O or CO₂ ; and N₂ or argon, flow through a lance or burner (18) tomix and react at the lance or burner tip 18a inside reactor vessel (20).Part or all of the solid uranium oxide product accumulates at (23) forperiodic discharge from the reactor vessel (20). Gaseous products andany entrained uranium oxide solids exit continuously at (22) into asuitable separating system (not shown). The reactor temperature may becontrolled by a combination of means for heating or cooling the reactorby jacket (21), control of the amount of O₂ in the feed stream (16), andpre-heat of the feed streams (16) and (17). Preheating the reactor byoperation of the reactor as a burner without UF₆ and use of an ignitiondevice (19) may also be employed as desired or if necessary to initiatethe process. Generally speaking, the temperature and composition of thefeed streams are such that when the two streams are brought together, aflame reaction occurs. If desired, however, the igniter (19) may bepositioned at the point where the gaseous streams come in contact toassist in the initiation of the reaction, although this is not usuallynecessary, particularly if the gaseous streams have been preheated.

FIG. 5 shows an alternative to the concentric tube arrangement of FIG.3. Thus, in FIG. 5, the reactant streams (24) and (25) are separatelysupplied through inlets (26) in nozzle means (28), the gases coming intocontact adjacent the nozzle discharge where they react to form UO₂ andHF as in the FIG. 3 arrangement.

The invention is illustrated by the following examples:

EXAMPLE 1

An experiment was done with gas feeds to a feed apparatus generally asdescribed in FIG. 1 except that the feed arrangement into the reactorcomprised three concentric tubes rather than two as shown. A mixture ofUF₆ at a rate of 250 g/hour and Ar was fed to an inner Al₂ O₃ tube. Amixture of water vapor at about 31 g/hour, H₂ at 19 std liters/hour andAr was fed into the annulus between the inner tube and a middle Al₂ O₃tube. Ar was fed into the annulus between the middle tube and an outergraphite tube. The assembly of tubes discharged into the reactor chamberat about 950° C. The uranium collected as solids beyond the tip of theinner tube with an estimated 0.1 second residence time between the endof the inner tube and the point of deposition of the uranium product.Analyses of the deposited solids by X-ray diffraction showed essentiallypure UO₂ without detectable amounts of fluoride compounds.

EXAMPLE 2

Thermochemical calculations were made using a computer program HSCChemistry for Windows 1.2, Outokumpu Research OY, FORI, Finland! todetermine the equilibrium compositions of products and the adiabatictemperature for reaction without heat transfer. The equilibriumcompositions versus temperature at essentially atmospheric pressure (1.0Bar) are shown in FIG. 2 for reaction of 1.0 mole of UF₆, 0.5 moles ofO₂, 1.4 moles of H₂ O, 2.2. moles of H₂ and 2 moles of Ar at atmosphericpressure. The equilibrium composition at 1100° C. is more than 99%conversion of the UF₆ to UO₂ and HF. With the feed gases pre-heated to100° C., the product temperature would be 1108° C. The same equilibriumcomposition shown in FIG. 2 would also result if the same total molaramounts of hydrogen and oxygen were fed using other combinations of H₂,H₂ O and O₂. However, the adiabatic temperatures would be less than 500°C. for no O₂ (i.e., all the oxygen as H₂ O) and over 2000° C. for no H₂O (i.e., all the oxygen as O₂). This illustrates how the composition ofthe products and the temperature without heat flux can be controlledseparately.

An important feature of the invention is that the UF₆ conversionreaction is completed quickly within a very short time and distanceafter the pre-heated UF₆ gas stream is mixed with the other reactant gasstream, e.g. a gas stream comprising H₂ O and H₂, possibly with an inertgas such as argon. Typically, the residence or reaction time is in theorder of fractions of seconds, for example, 0.1 second using a total gasvelocity at the tip of the feed lance or burner of about 3.5 feet persecond.

Without intending to be limited to any particular theory of operation,the invention appears to be based on a number of important factors.These include the finding that, using particular reaction conditions andprocedures, uranium oxide solids can be produced from a very rapid,one-step process for conversion of UF₆. Another important finding isthat, by using the conditions described herein, and the indicated feedconfigurations, for example, one pre-heated gaseous stream comprising amixture of H₂ O and H₂ and another preheated feed stream of UF₆, with orwithout oxygen and inert gas, conversion of UF₆ to essentially UO₂solids can be obtained without significant accumulation of UF₄, UO₂ F₂,or other uranium fluoride solids as intermediate compounds. Anotherimportant finding is that the adiabatic product temperature can becontrolled by controlling the temperatures and compositions of thefeeds.

It is known that UF₆ gas can be reacted with either H₂ or H₂ O gas togive useful conversions as follows:

    UF.sub.6 (g)+H.sub.2 (g)→2HF(g)+UF.sub.4 (s)        (1)

    UF.sub.6 (g)+2H.sub.2 O(g)→4HF(g)+UO.sub.2 F.sub.2 (s)(2)

The first reaction is used as the initial step for preparation ofenriched uranium metal from enriched UF₆. The UO₂ F₂ from reaction (2)is not directly a useful product but can be an intermediate forconversion to commercial nuclear fuels by reaction (3):

    UO.sub.2 F.sub.2 (s)+H.sub.2 (g)→2HF(g)+UO.sub.2 (s)(3)

This conversion reaction of a solid with a gas is more difficult thanreactions (1) or (2) where only gaseous reactants are involved. Reaction(3) commonly requires long times with good mixing of solids with anexcess of H₂. Reaction (1) might be used in combination with reaction(4) to prepare UO₂ from UF₆ :

    UF.sub.4 (s)+2H.sub.2 O(g)→4HF(g)+UO.sub.2 (s)      (4)

Reaction (4) is much less practical than reaction (3) because it is lessfavorable thermodynamically and the UF₄ from reaction (1) is a fusedsolid of low surface area.

According to the present invention, the reaction of an H₂ /H₂ O gasmixture with UF₆ gas at the specified conditions and feed compositionsresults in conversion of the UF₆ in a single process step to essentiallyuranium oxide UO₂ without formation of significant amounts ofintermediate uranium compounds. The overall conversion may berepresented by reaction (5):

    UF.sub.6 (g)+2H.sub.2 O(g)+H.sub.2 (g)→6HF(g)+UO.sub.2 (s)(5)

While reactions (1) or (2) (or other reactions) might occur on amolecular scale, the mixing of gases H₂ /H₂ O and UF₆, or modificationsthereof according to the invention, apparently allows reactions (3) or(4) to complete the conversion to uranium oxides while the intermediateproducts are still in a very finely divided (nearly molecular) state.

Thermochemical calculations can be used to identify the limitingrequirements for the foregoing reactions. The equilibrium compositionswere calculated for each uranium reactant and a ten percent excess ofthe other reactants. The results are given below as percentageconversions of the uranium feed over the most favorable range oftemperature at atmospheric pressure. The adiabatic product temperaturesfor these feeds pre-heated at 700° C. were calculated as an indicationof the heating or cooling requirements.

    ______________________________________                                                                   Resulting                                                                     Adiabatic                                                    Equilibrium Conversion                                                                         Product Temp. °C.                                     (for products in the indicated                                                                 for feeds at                                       Reaction No.                                                                            temperature range)                                                                             700° C.)                                    ______________________________________                                        1         -100% for 0 to 2500° C.                                                                 1885                                               2         >99% for 0 to 750° C.                                                                   1058                                               3         >98% for 850 to 1300° C.                                                                639                                                4         >98% for 850 to 1400° C.                                                                -273                                               5         >99% for 900 to 1300° C.                                                                1000                                               ______________________________________                                    

These thermochemical calculation results show that the reaction of UF₆(g) with a mixture of H₂ (g) and H₂ O(g) (reaction (5)) provides awell-controlled conversion with a good yield of UO₂ and HF. Theequilibrium conversions for reaction (5) are good over a 900° to 2000°C. range of temperatures. Reactants preheated to a reasonabletemperature (700° C.) give products within this temperature rangewithout reactor heat transfer for control. Additional excesses of H₂ orH₂ O feeds can be favorable to more complete conversions. The twoproducts are solid (UO₂) and gaseous (HF) over wide temperature ranges,thus allowing simple physical separation of the products.

The four reactions (1) to (4) are much less favorable in one or morerespects than reaction (5). Reaction (1) is very exothermic and willtypically give UF₄ as a vapor or liquid requiring cooling to prepare theUF₄ (s) for subsequent reaction to UO₂ by reaction (4) in a secondreactor. Reaction (4) is extremely endothermic and will require largeheat inputs to hold the favorable range of temperatures. Reactions (2)and (3) are mildly exothermic and endothermic, respectively. Butreaction (2) requires limiting the temperature to avoid thermaldecomposition of the UO₂ F₂ product. Reaction (3) must be carried outwith a large excess of H₂ to complete the reaction without thermaldecomposition of the UO₂ F₂ feed. Only reaction (5) can be carried outto give a solid product and high conversions without large heat fluxesor large excesses of H₂ O or H₂ feeds.

For the conversion of UF₆ by reaction (5), all the oxygen may besupplied as H₂ O(g). Thermochemical calculations indicate that the feedsare preferably preheated to 700° C. so the products can be at an optimumtemperature for complete conversion without requiring heat transfer fromthe reaction zone. The same conversion products could be formed withoxygen supplied by O₂ (g) as shown by reaction (6):

    UF.sub.6 (g)+O.sub.2 (g)+3H.sub.2 (g)→6HF(g)+UO.sub.2 (s)(6)

Reaction (6) is very exothermic and feeds at 25° C. result in adiabaticproduct temperatures over 2000° C. This high temperature is undesirablefrom a practical viewpoint and is less favorable thermodynamicallyprimarily because UF₄ (g) becomes a significant product.

The most practical conversion of UF₆ (g) to uranium oxides and HFinvolves supplying oxygen as both O₂ (g) and H₂ O(g) with theproportions controlled to control the product temperature. The reactionfor half of the oxygen from H₂ O(g) and half from O₂ (g) is:

    UF.sub.6 (g)+0.50.sub.2 (g)+H.sub.2 O(g)+2H.sub.2 (g)→6HF(g)+UO.sub.2 (s)                            (7)

Since large heat fluxes are not needed and temperatures are limited, thedesign of the equipment is simplified and a relatively wide variety ofmaterials of construction are acceptable for use. Thus, while ceramicmaterials such as alumina or graphite are generally preferred, theinvention permits operations wherein the reactor wall may besubstantially cooler than the reaction temperature, thus allowing theuse of metals such as nickel or Monel. Resistance heating elements onthe reactor wall may also be adequate. Alternatively, an inductionheating system may be used to heat the reactor.

Adiabatic product temperatures were calculated using ten percentexcesses of both oxygen and hydrogen with feeds at 25° C. and thedivision of oxygen supplied between O₂ (g) and H₂ O(g) as a variable.The results are:

    ______________________________________                                                                       Adiabatic Product                                                             Temperature for                                                               products of 6HF(g) +                           UF.sub.6                                                                              O.sub.2 H.sub.2 O                                                                              H.sub.2                                                                             UO.sub.2 + 0.2H.sub.2 O(g) +                   Moles   Moles   Moles    Moles 0.1H.sub.2 (g)                                 ______________________________________                                        1       0       2.2      1.1    370° C.                                1       0.15    1.9      1.4    637° C.                                1       0.30    1.6      1.7    891° C.                                1       0.45    1.3      2.0   1146° C.                                1       0.60    1.0      2.3   1389° C.                                1       0.75    0.7      2.6   1631° C.                                1       1.10    0        3.3   2143° C.                                ______________________________________                                    

The 1146° C. for UF₆ :O₂ :H₂ O molar ratio of 1:0.45:1.3 appears to benear the middle of the optimum temperature range. The amount of O₂ maybe controlled from a temperature measurement. Less O₂ is needed if thefeeds are preheated above 25° C.

Only two gaseous feed streams are needed since H₂ O and H₂ do not reactwith each other and UF₆ and O₂ do not react below 1000° C. A largeexcess of H₂ (e.g. greater than 50% theoretical) may be acceptable asthe excess can be recycled after condensation of the HF product. Theexcess of O₂ should be well-controlled and small (up to, for example,20% excess), particularly if an anhydrous HF product is desired tominimize the amount of water in the HF.

As will be evident from the foregoing, the process of the inventioncomprises reaction of two gaseous feed mixtures to give a one-step,efficient conversion of UF₆ into uranium oxide and HF. The gaseous feedcompositions and temperatures are controlled to give the optimumcompositions of uranium oxide and of gaseous products for recovery ofcondensed HF as a chemical of significant commercial value. In apreferred embodiment, one gaseous feed comprises UF₆, e.g. depleted UF₆,together with inert gas and/or part of the oxygen needed for theconversion. The other gaseous feed comprises hydrogen as H₂, H₂ O, NH₃or CH₄ and all or part of the required oxygen as H₂ O or CO₂. Increasingthe fraction of oxygen supplied as O₂ will generally increase thereaction temperature or the temperature of the products after reaction.On the other hand, increasing the fraction of oxygen supplied as H₂ O orCO₂ will generally decrease the temperature of the product afterreaction. The gaseous feeds may also be preheated before mixing toincrease the temperature of the products after reaction. This control oftemperature is important to simplify the design of process equipment.Avoiding excessive temperatures allows the use of nickel or other metalsinstead of ceramics. The control of temperatures without need for largeheat fluxes at the reaction zone is also an important advantage orsimplification.

Specific features of the invention include the following:

(1) The process provides uranium oxides from UF₆ which have usefulproperties sufficient to meet the requirements for storage, disposal oruse of uranium.

(2) The process provides a one-step conversion of UF₆ into uranium oxideand HF. The UF₆ starting material and the other reactants are fed into areaction zone to give readily separable solid and gaseous products.

(3) The process involves the feed of two or more gaseous feeds which areseparately stable with compositions and temperatures that give afavorable thermodynamic equilibrium composition and temperature withlittle or no heat transfer during the reaction. One gaseous feed is theUF₆ with varying amounts of O₂ and/or inert diluent gas, such as Ar. Theother gaseous feed includes one or more sources of hydrogen (H₂, NH₃, H₂O or CH₄) and one or more sources of oxygen (H₂ O, CO₂). The compositionof each gaseous feed stream, the ratios of the two feed streams, and thetemperatures of the feeds can be controlled and varied to provide thefavorable thermodynamic equilibrium compositions and temperatures.

(4) The preferred reaction product temperatures for conversion of UF₆into uranium oxides and HF are most commonly in the range of 900° to2000° C.

(5) The preferred reaction pressure for conversion of UF₆ into uraniumoxide and HF is commonly in the range of atmospheric, but elevatedpressures can be accommodated.

(6) The process results in a gaseous product composition that can becontrolled to allow recovery of HF for commercial use.

(7) The reaction temperature may be controlled by controlling thefractions of oxygen fed as O₂ and fed as H₂ O or CO₂. Increasing thefraction added as O₂ increases the temperature of the reaction products;increasing the fraction fed as H₂ O or CO₂ decreases the temperature.This control of temperature prevents excessive temperatures, minimizesor eliminates the need for heat transfer to and from the reaction zoneand provides an optimum temperature for obtaining the compositions ofproducts desired.

(8) The product compositions can be controlled and varied by varying theproportions of total oxygen to UF₆ and total hydrogen to UF₆ in thefeeds. Since the oxygen in the feed can be from O₂ and H₂ O or CO₂ andthe hydrogen can be from H₂ and H₂ O, the control of oxygen/UF₆ andhydrogen/UF₆ ratios can be independent of the control of reactiontemperature as described in (7).

(9) Control of temperature can also be accomplished by a controlledpreheating of the gaseous feeds. For example, feeds of UF₆ (without O₂)and H₂ +H₂ O can be preheated to about 700° C. to give an adiabaticreaction temperature within the preferred range of temperatures.

(10) The feed apparatus and procedures allow the gaseous feeds to be fedand reacted directly to final product without excessive corrosion orerosion by the feeds and without handling and plugging problems causedby bulk solids of uranium intermediates such as UO₂ F₂ and UF₄. Theconversion of UF₆ to uranium oxides is completed without need for amixing of a gaseous feed with uranium solids of an intermediatecomposition (e.g. UO₂ F₂ or UF₄). The mixing and reaction of a gas withbulk solids is commonly slower and more difficult than the mixing andreaction of gases. The elimination of such a process operation is amajor difference between the present invention and prior art procedures.

(11) The two principal products in the present process are a solid(uranium oxide, primarily as UO₂) of very low volatility and gaseous HFwhich can be condensed when cooled. This allows easy and efficientseparation of the uranium oxide product and the HF product by simplephysical separation (e.g., filtration).

(12) The control of reaction temperatures and heat fluxes by controllingfeed composition (more specifically, the oxygen content of the UF₆ feed)greatly simplifies the design of the process equipment. Some materialsof construction and temperature control procedures are more practicalfor the process described than for highly exothermic or highlyendothermic process reactions. Mildly exothermic process reactions mightbe contained by cooled metal reactor walls without the need forceramics. Low heat fluxes through the reactor walls allow simpler heattransfer and temperature control designs.

(13) The process may be carried out continuously under substantiallyadiabatic conditions with the oxide solids and gaseous HF periodicallyor continuously removed from the reactor.

It will be appreciated that various modifications may be made in theinvention as described above without deviating from the spirit and scopethereof as defined in the following claims wherein:

What is claimed is:
 1. A single-step process for producing solid uraniumoxide and gaseous HF from UF₆ which comprises bringing together twogaseous reactant streams wherein at least one of the streams ispreheated to a temperature of 700°-1000° C. prior to bringing together,one of said streams comprising UF₆ optionally admixed with oxygen as O₂,and the second reactant stream comprising a mixture of hydrogen as H₂ oras a hydrogen-containing compound and oxygen as an oxygen-containingcompound, said gaseous reactant streams being brought together in areactor at a temperature and composition such that the UF₆ is convertedrapidly by flame reaction into readily separable solid uranium oxide anda gaseous HF product.
 2. The process of claim 1 wherein the reaction iscarried out at essentially atmospheric pressure.
 3. The process of claim1 wherein the UF₆ is depleted UF₆ and the uranium oxide productcomprises UO₂.
 4. The process of claim 1 wherein the solid uranium oxideis separated from the gaseous HF product and the gaseous HF product iscondensed to recover HF.
 5. The process of claim 1 wherein the gaseousfeed streams provide a reaction product having a temperature within therange of 900° C. to 2000° C.
 6. The process of claim 5 wherein bothgaseous feed streams are preheated to a temperature sufficient to yieldproducts in the temperature range of 900° C. to 2000° C.
 7. The processof claim 1 or claim 6 wherein the reactor is heated by external means toa temperature sufficient to yield products in the temperature range of900° C. to 2000° C.
 8. The process of claim 1 wherein the gaseous feedstream comprises UF₆ and O₂ and the temperature of the reaction productis controlled by controlling the amount of O₂ in said UF₆ feed stream.9. The process of claim 1 wherein the ratio of hydrogen or oxygen to UF₆is regulated to control the amount of H₂ O vapor in the gaseous HFproduct.
 10. The process of claim 1 wherein one or both of the feedstreams also includes an inert gas.
 11. The process of claim 1 whereinone gaseous feed stream comprises UF₆ and O₂ and the other gaseous feedstream comprises NH₃ and H₂ O.
 12. The process of claim 1 wherein onegaseous feed stream comprises UF₆ and O₂ and the second gaseous feedstream comprises H₂ and H₂ O.
 13. The process of claim 1 wherein onegaseous feed stream comprises UF₆ and O₂ and the second gaseous feedstream comprises H₂ and CO₂.
 14. The process of claim 11, 12 or 13carried out at atmospheric pressure.
 15. The process of claim 1 whereinthe gaseous reactant streams are reacted at atmospheric pressure and atemperature of 900° to 2000° C. for less than a second.
 16. The processof claim 15 wherein the process is carried out continuously withcontinuous removal of solid uranium oxide and gaseous HF product. 17.The process of claim 15 wherein the reaction is carried out atessentially atmospheric pressure.
 18. The process of claim 1 wherein thereaction is carried out at a pressure above atmospheric but below 250psia.
 19. The process of claim 1 wherein the reaction is carried out ata pressure slightly below atmospheric in the range of 5 to 13 psia. 20.The process of claim 1 wherein the solid uranium product is separatedfrom the HF by physical means internal to the reactor.
 21. The processof claim 1 wherein the solid uranium product is separated from the HF byphysical means external to the reactor.
 22. The process of claim 1 wherethe oxygen and hydrogen feed gas proportions are near the stoichiometricamounts to fully convert UF₆ to UO₂.
 23. The process of claim 1 wherethe oxygen and hydrogen feed gas proportions are in excess ofstoichiometric but generally less than 1.5 times the stoichiometricamounts.
 24. The process of claim 1 wherein the oxygen and hydrogen feedgas proportions are less than the stoichiometric amounts required tofully convert UF₆ to UO₂.